1. Field of the Invention
This invention generally relates to a fixing device for fixing a developed toner image to its underlying support medium with the application of heat, and, in particular, to a heat-fixing device which is suitable for use in an electrophotographic copying machine. More specifically, the present invention relates to a heat-fixing device including a heat plate along which a transfer medium bearing thereon a transferred toner image is transported whereby the toner image becomes fixed to the transfer medium by fusing.
2. Description of the Prior Art
In a typical electrophotographic copying machine of the wet-development type, a liquid developer comprised of carrier liquid and charged color particles, or commonly called toner particles which are dispersed in the carrier liquid is supplied to the surface of a photosensitive member on which an electrostatic latent image is formed, so that the toner particles are selectively attracted to the photosensitive member thereby the latent image is developed and a visible toner image is formed. Then a transfer medium, which is usually plain paper, is brought into contact with the photosensitive member and the toner image is transferred to the transfer medium as it is peeled off the member. Thereafter heat is added to the transfer medium now bearing thereon the transferred toner image so that the carrier liquid impregnated into the transfer medium evaporates and the toner particles are fused to become fixed to the transfer medium.
Stated more in detail with respect to a fixing process of a toner image, the transfer medium bearing thereon the transferred toner image on its front surface is transported in contact with and along a heat plate which is heated to a predetermined temperature, e.g., 200.degree. C., by means of a heater provided in the heat plate. Thus, as the transfer medium moves along the heat plate, it becomes heated by receiving heat from the heat plate through its back surface and therefore the carrier liquid absorbed in the transfer medium evaporates and the toner particles are fused to become fixed to the transfer medium. However, a wait time period is usually required for the heat plate to reach a steady-state condition after power-up of the copying machine. Particularly, when the power switch of the copying machine is turned on after a long resting period, e.g., overnight, it often takes nearly two minutes for the heat plate to become heated to a predetermined temperature, which can be a cause for impairing practicality and usefulness of the copying machine.
It is true that even if the temperature of the heat plate is lower and the transfer medium having thereon a transferred toner image which is not completely fixed is discharged to a tray, the toner image may become completely fixed by natural evaporation of the carrier liquid. However, in this case, abrasion of the transfer medium would cause disturbance or removal of the toner particles. Moreover, since the toner particles are fixed to the transfer medium without being fused, the resulting image is low in quality, e.g., non-uniform in density.
From the power capacity of the copying machine, there is a limit in the amount of power which may be supplied to the fixing device so that there is a ceiling for the amount of electric power which may be applied to the heat plate. To cope with this situation, the thermal capacity of the heat plate may be made smaller thereby providing an increased speed for the heat plate to arrive at a predetermined temperature. In this case, however, if a number of transfer mediums are transported across the heat plate one after another continuously, the fixing performance of the heat plate will fluctuate to produce fixed images of poor quality because of thermal unstability caused by a reduction in thermal capacity. Accordingly it is not advantageous to reduce the thermal capacity of the heat plate because it will create unstability in performance under the steady-state condition.